Theres a chance it could turn up still, these things have a tendency to turn up. could have been put in with a box collection by mistake or dropped anywhere and a customer could hand it bak in etc.
Did he sign it back into the locker? I know you said it wasn't scanned in but he may have still signed the special book to say how many specials he handed back
I had a business hand me a 739 endorsed signed for letter that had been mixed in with their mail. Luckily the date on it was the day before so it may not even have been missed yet.
We have a 'paranoid book' when we bring stuff back. Most of us write every packet address in it, and if recorded/special peel off the tiny number sticker and place it next to the address in the book or if tracked write the 13-digit number in. If you just do that for the specials/recordeds and tracked (as it's busy) it ALWAYS covers your ar$e.
In our DO you just put them in the cage when you get back and they scan them when they've got time. Some days there's piles of them waiting.
We do something similar, tracks, recorded & normal parcels that have been P739'd go into cages outside collection area & are sorted whatever way they sort them, specials get either handed over to someone through window hatch or left sitting at the hatch for the staff there to scan as ready for collection or ready for re-delivery.
Only scanning postmen do of the item is either delivered, or not delivered.
We don't even sign for them ,we are just told our total signable items and walk in the den and pick them up.There is no security, anyone could take anything and I often wonder how they could prove the postie lost an item when he may never have had it.